The general purpose of screens is to prevent the ingress of insects, while providing ventilation. A typical screen assembly is made up of screen cloth, fabric, or mesh attached to a screen frame in a manner discussed in more detail below. For brevity, the term “screen” is used herein, and includes such screen cloth, fabric, mesh or similar ventilation material.
Screen frames for windows, doors, operable skylights and the like are commonly made of four elongated frame members, called screen bars, of uniform cross section. These bars are typically roll-formed from aluminum or sheet steel, although some may be extruded aluminum. (Plastic and wood are also used, but to a lesser extent.) These screen bars are supplied from the screen bar manufacturer in lineal form and are cut to a final length by the screen assembly manufacturer. Further, these screen bars are held together at the corners with plastic or metal inserts, called corner keys, to form the screen frame.
Different style corner keys are available and are designed to match the particular screen bar used. The most popular corner key allows the screen bar to be cut straight at 90° at the ends. These keys typically are made from injection molded plastic and have a square block body to visibly fill the corner area of the frame. Attached to the body are insertion prongs that are pushed into the hollow screen bar profile to create friction fit connections. Corner keys requiring a 45° miter cut on the ends of the screen bar also can be used. These keys, usually metal, are less expensive and entirely hidden inside the screen bar. These keys also provide a friction fit connection.
Screen is then affixed to the screen frame, in a manner discussed below, to form a screen and frame assembly. These assemblies are then removably secured to windows, doors (e.g., patio screen doors), operable skylights, and the like. Screen and frame assemblies for such openings are very similar, often differing only in size. Accordingly, for brevity, screen and frame assemblies for windows are described herein. Nevertheless, it will be understood that this discussion applies equally to screen and frame assemblies for doors, operable skylights and the like.
It is desirable that the screen be a light-weight fabric or mesh, and stretched taut across the screen frame to avoid unsightly sag and to allow a viewer to see through the screen with minimal visual interference. However, if the screen is tensioned excessively, the screen bars deform inwardly in an hourglass shape. This resultant shape is not only aesthetically undesirable, but also can prevent proper installation in the window opening. Excess screen tension also increases the risk of tearing the screen during manufacture of the screen and frame assembly or while the assembly is in service.
Typically, the screen is fiberglass yarn or roving, which is coated, for example, with polyvinyl chloride (PVC), woven and heat fused. The next most popular form of screen is made by weaving drawn aluminum wire, which is subsequently painted. The PVC coated fiberglass screen is the most popular type, by approximately a 4 to 1 ratio (in area). However, both offer the desired attributes of suitable strength and an open weave.
To compensate for deformation of the screen frame into the hourglass shape discussed above, generally the screen bars are manufactured with an outward bow, in the plane of the screen, before the screen is installed. After the screen is installed into the screen bar by the manufacturer, its final tension straightens the frame members in the final assembly. This “pre-bow” is set into the screen frame during the extrusion or roll forming process to make the screen bar lineal.
Typically, roll-formed bar has approximately 20 millimeters (0.75 inches) of bow over a 3.7 meter (12 feet) length. Additional bow is usually set by hand into the roll-formed bar prior to screen installation when the length of the frame members is greater than 1 meter (approximately 3.5 feet). Pre-bowing is not generally required, however, when the screen bar is sufficiently rigid to resist deformation caused by the resultant screen tension.
It is the current practice, essentially industry-wide, to secure screen in open grooves formed along inside edges of the screen frames using a stuffer strip known as “spline” and its associated fastening techniques. The open grooves are known as “spline grooves.” A spline is often a wire-like, extruded rigid plastic or foam material, although some splines are made from metal, especially for use with aluminum screens. A spline is usually round or T-shaped in cross section, but can be U-shaped, for example.
U.S. Pat. No. 5,039,246 (the '246 patent) shows a conventional method of securing screen to a frame member using a spline. Using the reference numerals of the '246 patent, the spline 58 is forced into a spline groove or recess 56 in the screen bar 20, with the screen 22 sandwiched between the spline 58 and the spline groove 56.
The screen 22 is held by friction between the spline 58 and the spline groove 56 with the resulting interference fit. A lip 50 and a ledge 52, part way down one side of the groove wall, are typically included to help trap and improve the strength in retaining the screen 22. The spline 58 and trapped screen 22 are forced into the groove 56, usually by hand, with the use of a roller device 70, including a roller 72. The term, “hand wiring”, is used to describe the action of securing the screen 22 with the spline 58 into the spline groove 56. Many attempts have been made to automate the installation of spline by machine. However, this automation has proven to be very difficult and machines of this nature have not been widely accepted as a viable option to hand wiring.
The conventional procedure for manufacturing and hand wiring a screen and frame assembly is discussed in more detail below. First, the screen bars are cut to length:, accounting for the corner key dimensions. Then, the screen frame is assembled using the cut screen bars and corner keys. As discussed above, when light construction screen bars are used, as is normally the case, a balance between pre-bow tension and screen tension is necessary to ensure straight screen bars and desirable tension in the final assembly. When the screen bar has insufficient pre-bow tension, the bars are deformed by hand a sufficient degree after the corner keys have been inserted. As discussed above, the amount of pre-bow is determined based on experience, but is typically a few millimeters of bow per meter length of the screen bar.
The screen frame is then secured to a table using locator (stop) blocks, which prevent shifting and maintain the frame square during screen installation. The table typically has permanent stop blocks for orienting the screen frame. If the screen bar is not constrained, when the spline is inserted into the screen bar, excessive tension may be placed on the frame, causing the frame to hourglass inwards. To avoid hourglassing, removable blocks are located on the inside of the frame segment to limit deflection of the screen bar by the screen tension on assembly. (The spline groove must be facing up and unobstructed by the blocks.) More elaborate tables use removable blocks arranged in grooves cut into the table, with the removable blocks being secured by integral friction clamps. To avoid the need for blocking to prevent hourglassing, some manufacturers use extruded screen bar, instead of roll-formed screen bar, because of the greater strength of a (thicker) extruded section.
After the screen frame is secured to the table, the screen is pulled from a roll and positioned to cover the opening formed by the frame. Ideally, no excess screen is used, but this is difficult to achieve in practice. As a result, most manufacturers cut the screen approximately two inches wider than the frame width, so that the screen is pulled past the end of the frame by approximately one inch to ensure that sufficient amount of screen can be rolled into the spline groove along the frame perimeter. In either technique, the screen is positioned over, with edges parallel to, the secured screen frame.
The screen and spline are installed into the spline groove by starting in one of the frame corners. The screen is then pulled taut at the next corner with one hand, keeping it straight and parallel to the edge of the mating screen bar. The spline is simultaneously held above the groove in the same manner as the screen, with the same hand. With the other hand, the installation roller is pushed along towards the upcoming corner with a firm downward force to push the spline and trap the screen into the spline groove. This action is repeated on the second and third screen bars. On the last screen bar, most of the tension is set into the screen. On this leg, the screen is pushed into the screen bar with the installer's finger, just prior to the insertion of the spline. This pre-insertion technique reduces the final tension in the screen to the desired level. The spline is cut at the final corner with a utility knife.
After the spline and screen are inserted in all screen bars, excess screen around the edge of the frame is cut away with a utility knife. To do this, the point of the blade is pushed against the screen bar, through the screen, immediately adjacent to the spline groove around the outside edge of the screen bar. Care must be taken to cut the screen close to the spline groove without cutting the screen covering the opening formed by the frame. The finished screen and frame assembly is removed from the table, inspected, and any necessary hardware is attached.
The current hand wiring process using spline has several drawbacks, however.
Current standards for screen and frame assemblies are established by associations such as the Screen Manufacturers Association (ANSI-SMA SMT 31-1990) in the United States and the General Standards Board in Canada (CAN-CGSB-79.1-M91). These standards cover particular elements of screen and frame assemblies for windows, patio doors and the like. For example, these standards set forth tolerances in terms of the strength of the screen, the strength required to fasten the screen to the screen bar, the amount of sag in the screen, etc. Although these standards generally can be met by using the spline technology discussed above, very close and consistent dimensional tolerances are required between the spline and the spline groove, respectively, in order to achieve the specified fastening strength. These tolerances require close attention and skill with current screen bar roll-forming and extrusion technology and current spline hand wiring techniques. Any out-of-tolerance spline and screen bar produced costs the manufacturer in wasted time, material and goodwill.
Further, the amount of force required by an installer to secure the screen with the spline in the spline groove may be high enough to cause repetitive strain injury, e.g., carpal tunnel syndrome, to one who routinely performs this job. This is of major importance, since this type of injury is serious and has recently received heightened public awareness. Further, such an injury to an installer is also costly to the manufacturer in terms of compensation and loss of skilled labor.
Also, the hand wiring technique is particularly difficult and time-consuming. Notably, it is difficult to control the wire-like spline material and simultaneously control the screen tension with one hand, while the spline is rolled in with the other hand. This operation requires a high degree of skill and careful attention. This adds to the final manufacturing cost, and, hence, increases the final cost to the consumer. Final product consistency is difficult to maintain.
Quality control also has become an issue with current spline techniques. Specifically, installers have learned ways to make their jobs easier, to the detriment of quality control. This is particularly true when using PVC spline. For example, an installer will stretch the PVC spline just prior to insertion, in order to reduce the diameter of the spline. This, of course, makes it easier to install. However, this also reduces the “pull-out” force or attachment strength of the spline and screen. The result is that the screen can be more easily pulled out from the spline groove, which is undesirable. (This, however, is not an issue with polyethylene spline, which does not stretch in the manner of PVC spline.)
There are other drawbacks associated with conventional spline techniques. In particular, the use of a separate fastening device, such as a spline, requires separate inventory control and associated costs. Screen manufacturers prefer to minimize inventory. Therefore, it is desirable to eliminate the spline as a separate item. Also, the need to have a strong interference fit in securing the spline necessitates stiff walls on the spline groove. Further, the spline technology makes the design of automatic assembly equipment extremely complex.
For the foregoing reasons, a need has arisen to provide a screen and frame assembly that eliminates the requirement of a spline. An additional need has arisen to manufacture screen products more easily.
Some attempts have been made in the art to provide screen and frame assemblies without a traditional spline. For example, in U.S. Pat. No. 3,255,810, a continuous strip of fusible material is fused with the screen material and then inserted into the groove in the frame. In U.S. Pat. No. 4,568,455, the bonding of a screen to a thermoplastic frame is accomplished by resistance heating of the screen using an electrical potential of four volts and a current of approximately, 2,200 amps, which is applied for approximately forty-eight seconds, to fuse the thermoplastic. This method, however, requires external tensioning until the thermoplastic cools and solidifies.
In another aspect, U.S. Pat. No. 4,968,366 teaches a complex method of manufacturing tension screens using an apparatus that includes a screen tensioning frame and a platform positioned adjacent to the tensioned screen. The platform includes heating elements about the periphery of a sheet heater. The heating elements receive a screen frame which can be lifted into contact with the screen in the tensioning frame. The screen cloth is pre-tensioned by an external frame. The screen frame is heated to thermally expand the screen frame. Then the screen cloth is expanded by heating, by an amount substantially equal to the amount of thermal expansion of the screen frame during the step of heating the screen frame. Next, the expanded and pre-tensioned screen cloth is bonded to the heated screen frame. The screen frame is then cooled by blowing air over the screen frame. The heat of the screen cloth is maintained by shielding the bonded screen cloth from the blowing air and heating the bonded screen cloth concurrently, while cooling the screen frame, so that the screen cloth does not cool faster than the screen frame during cooling of the screen frame.
Thus, in the arrangement of U.S. Pat. No. 4,968,366, it is necessary to heat the entire mating surface, while the screen is maintained under high tension, and to match, or compensate for, the different thermal expansions of the frame and screen cloth. This complex technique requires high manufacturing precision, including proper tensioning of the screen and mating of the heating elements and the tensioning frame. Further, this technique is too slow and cumbersome to be considered practical for the manufacture of screen and frame assemblies for windows and the like.
Other techniques, in general, are known to fuse screening material to frames. For example, U.S. Pat. No. 4,675,065 (the '065 patent) shows a method for securing a microsieve to a support member. A laser beam is directed against a point on the upper edge of a well which contains the microsieve to melt fusible material in contact with the laser beam. The laser-melted fusible material travels down the well wall, contacts the edge of the microsieve and solidifies to secure the microsieve. Japanese patent document No. 63-137828 (the '828 document) shows a single step method of ultrasonically welding screening net to the bottom of a small, cylindrical container using resin and a single, vibrating tip, which is identical in size to the container bottom. The exotic techniques for the small parts, as described in the '065 patent and the '828 document, are generally limited to their particular applications.
Accordingly, a need has arisen for a screen and frame assembly for windows, doors and the like in which the screen is secured to the frame quickly, with reduced manual labor.